Frequency selective amplifier



FREQUENCY SELECTIVE AMPLIFIER Filed Oct. 11, 1945 ALLEN E. HASTINGS QW LW' Patented Dec. 13, 1949 UNITED STATES PATENT QEFlCE FREQUENCY SELECTIVE AMPLIFIER Allen E. Hastings, Washngton,'D. C. Application October 11,'1945, Serial N0. 621,667

4 claims'. (o1. 179-171) l A( Granted under the act of March' 3, 1883, as

This invention relates in general to a frequency selective amplier and in particular to a tunable negative feedback amplifier.

An object of this invention is to provide a tunable, negative feedback amplifier, having a widely variable frequency-selectivity characteristic. Another object of this invention is to provide a tunable amplier, having a constant Q char acterstic throughout its tuning range. Other objects and features of the present invention will become apparent upon a careful consideration of the following detailed description when taken together with the accompanying drawings.`

Fig. l is a schematic diagram showing in general form a portion of the circuit shown in Fig. 2, and

Fig. 2 is a detailed circuit diagram of a preferred embodiment of the invention.

In constructing a frequency selective amplifier in accordance with the teachings of the invention the operating characteristic of negative feedback is combined with that of a unique fre.-

quency-selective feedback network to produce the desired results. In general, the feedback network and amplifier are arranged so that at a selectable frequency the feedback voltage is zero andY hence the gain of the amplifier is a maximum. At any other frequency the output of the feedback network will be such that the gain of the amplifier will be substantially unity. Tuning of the amplifier is accomplished by adjustment of the frequency response characteristic of the lfeedback network.

Specifically the feedback network of the invention is a four-terminal, four-leg, tunable resistance-capacitance bridge circuit as shown in Fig. 1. An inspection of Fig. 1 reveals that the bridge circuit illustrated therein is provided with four distinct terminals I, 2, 3 and 4 connected together by four distinct impedance legs. The legs connecting terminals I and 3, and 2 and 3 are pure resistances NR' and R' respectively and therefore represent impedances which do not vary with frequency. The leg connecting terminals I and 4 comprises a series connection of resistance and capacitance while that connecting terminals 2 and 4 comprises a shunt connection of resistance and capacitance. Therefore, the latter two legs represent impedances which vary with frequency. For instance, at zero frequency the impedance of the leg connecting terinfinite frequency the impedance of the leg conamended April 30, 1928; 370 0. 757) necting terminals I and 4 is simply R1 While that of the leg connecting terminals 2 and 4 is equal to a short circuit due to shunt capacitor C2. Therefore, since the legs connecting terminals I and 3, and 2 and 3 are pure resistances which do not vary with frequency while the legs connecting terminals I and 4, and 2 and 4 represent impedances which vary with frequency, the voltage e' developed across the terminals 3 and Il formed by the junction points of the xed and variable impedance legs respectively will be zero when a voltage of magnitude e and frequency w is impressed across terminals I and 2. The angular velocity w at which this null in voltage e' is zero depends upon the circuit con- Istants R1, C1, R2 and Cz and may be calculated 'by the use of Kirchboifs laws, from which the following equation is derived:

(I) (w2R1R2C'1C2-1) +7`w(NR2C1-R1C1R2C2) =0 For this expression to equal zero the real and imaginary components must equal Zero. Therefore, the output voltage e from the bridge is at a null, when In practice it is found most convenient to make when and

It follows that if either R1 and R2 or C1 and ',Cz are made variable and are ganged the frequency at which a null will occur in the output .voltage e can be varied inversely with R or C.

It also follows that by using a bridge-controlled ,feedback network of the foregoing type, arranged so that the feedback voltage is derived from across the output terminals t and 4 a highly selective and tunable amplifier results. Further.- more, from Equation I it is apparent that .if

N,for in eect one of the xed resistance legs connecting terminals I and 3cr 2 and 3*., is made variable the phase angle of the feedback com;-

ponent e can be varied and hence the selectivity of the amplifier can be varied. Specifically it has been found that the selectivity of the amplier is a function ,l and N as expressed in the following equation:

ance 6 and the composite resistance formed by fixed resistance 8 and variable resistance I correspond respectively to the bridge legs NR' and R' connecting terminals I vand 3, and2 and 3 of Fig. 1. Thus terminal I exists at the plate of tube 5, terminal 2 at the cathode of tube and terminal 3 at ground. Resistance 'I is made variable so as to provide a convenient adjustment of N and hence an adjustment of the selectivity of the amplifier. As connected the bridge is driven in push-pull, that is, a signal applied to the grid of tube 5 will appear uninverted across the cathode load While that appearing across the plate load resistance S will be inverted, so that as terminal I swings negative terminal 2 Will swing positive and vice versa.

Variable resistances 9 and Il) correspond respectively to resistance elements R1 and R2 of Fig. 1. Smaller variable resistances II and I2 provide a means for adjusting the minimum frequency set"- tng of the amplifier. Condensers I3 and I4 colrespond respectively to condensers elements C1 and C2 of Fig. l and may be of the plug in vtype to facilitate extending the operating frequency range of the amplifier. Y

For optimum performance of the circuit it is desired that the amplifier provide constant gain over its tuning range. To this end a direct-coupled amplifler circuit is employed. Tube com` ponent I9 represents the input stage of the amplifier to which signals applied at input terminals 24 are communicated. Tube component I9 is provided with a cathode load resistance I8 which also serves as the cathode resistance for the second tube 2l in the circuit. Tube l2I is an ordinary class A amplifier which is provided with a suitable plate resistance 25 and a decoupling ilt'e'r comprising capacitor 22 and resistance 23. The

way of cathode resistance i8 to tube v2l where lt will appear, again without inversion, at the plate of tube 2 l. The output from tube 2i 'applied in parallel to the output terminals I5 fof the circuit and to the grid of tube '5. A

The output from the bridge circuit 4is taken from across terminals 3 and 4 and connected through a suitable coupling circuit comprising capacitor it' and resistance vlli to the grid of tiibe 2l. To prevent excessive loading on the bridge, resistance i6 is made as high as permissible.

As thus arranged a signal of the desired frequency, that is, a signal of the frequency that corresponds to the null frequency, as adusted by rcsistances 9 and I 0, of the bridge will appear at the output terminals I5 of the circuit as a product of the maximum gain of the amplifier and the input voltage, since the feedback voltage existing across terminals 3 and 4 at this frequency is zero. A signal of lower frequency than the null frequency of the bridge will cause a voltage to exist across the output terminals 3 and 4 of the bridge which when fed back through capacitor I1 to the grid of tube ZI will cause a greatly reduced signal to appear at the output terminals I5. The magnitude of an output signal -at terminals i5 depends upon the selectivity of the amplifier (setting of resistance 1) and the frequency separation between the signal in question and the null frequency of the bridge. At signal frequencies above the null frequency of the bridge a similar reduction in signal gain takes place.

In analyzing the operation of the circuit it will be recognized that at frequencies considerably below the bridge null frequency the impedance of the bridge leg connecting terminals I and 4 Vwill be considerably greater, due to the high reactance of capacitor i3, than the impedance of 'the bridge leg connecting terminals 2 and 4. Thus, the phase of the voltage appearing across the terminals 3 and will be that which is developed across the cathode load resistance of tube 5. Since this voltage is of the 'same phase as that which appears at the plate of tube 2l it will produce negative feedback' when coupled to the grid of tube 2i. Similarly at frequencies considerably above the bridge null frequency, the impedance of the bridge leg connecting terminals I and II will again be considerably greater than that of the leg connecting terminals 2 and 4. Thus the phase of the voltage appearing across terminals 3 and 4 will again be that which is developed across the cathode load resistance of tube 5 and again of the proper phase to cause negative fedback. Although I have shown and described only `a certain and specific embodiment of the invention it is to be understood that I ain fully aware of the many modifications possible thereof. Therefore, this invention is not tobe restricted except insofar as necessitated by the spirit of the prior V'art and the scope of the appended claims.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment 'of any royalties thereon or therefor.

What is claimed is:

1. A .frequency selective amplifier, comprising a four terminal, four leg frequency selective bridge network, two of thel four legs of the `bridge being formed b y a pair of resistances connected across the input terminals of said bridge, the circuit constants of said bridge network being vselected to provide substantially zero signaltransmission at a selectable frequency, an electron discharge device having an anode, a cathode and a control grid, means connecting the input terminals of said bridge between the .anode and cathode of said discharge device whereby one off said pair of resistances forms the anode load and the other the cathode load for the discharge device, an amplifying means, means coupling a signal component from said amplifying means to the control grid of said discharge device, and means .feeding the .output from said bridge .back

into said amplifying means at a point therein to produce degenerative feedback.

2. A frequency selective amplifier, comprising a four terminal, four leg frequency selective bridge network, two of the four legs of the bridge being formed by a pair of resistances connected across the input terminals of said bridge, the circuit constants of said bridge network being selected to provide substantially zero signal transmission at a selectable frequency, an electron discharge device having an anode, a cathode and a control grid, means connecting the input terminals of said bridge between the anode and cathode of said discharge device whereby one of said pair of resistances forms the anode load and the other the cathode load for said discharge device, an amplifying means, means coupling a signal component from said amplifying means to the control grid of said discharge device, means feeding the output from said bridge back into said amplifying means at a point therein to produce degenerative feedback and means incorporated in said bridge network for adjusting the phase of the feedback component to said amplifying means to thereby vary the selectivity of said amplifier.

3. A frequency selective amplifier, comprising a four terminal, four leg, frequency selective bridge network, two of the four legs of the bridge being formed by a pair of resistances connected across the input terminals of said bridge, one of said resistance legs being substantially twice the value of the other, the circuit constants of said bridge network being selected to provide substantially zero transmission at a selectable frequency, an electron discharge device having an anode, a cathode and a control grid, means connecting the input terminals of said bridge between the anode and cathode of said discharge device so that said one of said resistance legs forms the anode load and the other the cathode load for said discharge device, an amplifying means, means coupling a signal component from said amplifying means to the control grid of said discharge device, and means feeding the output from said bridge back into said'amplifying means at a point therein to produce degenerative feedback.

4. A frequency selective amplier, comprising a four terminal, four leg, frequency selective bridge network, two of the four legs of the bridge being formed by a pair of resistances connected across the input terminals of said bridge, one of said resistance legs being substantially twice the value of the otherVthe circuit constants of said bridge network being selected to provide substantially zero transmission at a selectable frequency, an electron discharge device having an anode, a cathode and a control grid, means connecting the input terminals of said bridge between the anode and cathode of said discharge device so that said one of said resistance legs forms the anode load and the other the cathode load for said discharge device, an amplifying means, means coupling a signal component from said amplifying means to the control grid of said discharge device, means feeding the output from said bridge back into said amplifying means at a point therein to produce degenerative feedback and means for varying the phase of the feedback component to thereby vary the selectivity of said amplifier, said last named means comprising means for adjusting the value of the resistance forming the cathode load for said discharge device.

ALLEN E. HASTINGS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,941,384 Bowles Dec. 26, 1933 2,173,426 Scott Sept. 19, 1939 2,390,824 Berry Dec. 11, 1945 2,411,706 Berkoi Nov. 26, 1946 

